In steel smelting, a method of charging iron ore containing iron oxide into a blast furnace along with a reductant such as coke, heating and melting the iron ore under a reducing atmosphere to obtain crude steel, and refining the crude steel in a converter to obtain desired steel has been used.
The iron oxide that is a raw material of the steel is a limited resource, and furthermore it is gradually hard to obtain high-quality iron ore required to maintain a quality of steel.
Meanwhile, with respect to nickel becoming a raw material of stainless steel, technology for smelting low-grade oxide ore as a raw material due to a tendency toward resource exhaustion of sulfide ore that has been used in the past has been developed and put to practical use.
To be specific, nickel oxide ore such as limonite or saprolite is put into a pressure device such as an autoclave along with a sulfuric acid, and nickel is leached under high pressure and high temperature of about 240 to 260° C.
The nickel leached into a solution of the sulfuric acid is used as nickel metal or a nickel salt compound by adding a neutralizer to neutralize a surplus acid, separating a leach residue by solid-liquid separation, separating impurities to recover the leach residue as an intermediate raw material in the form of hydroxide or sulfide, and further refining the intermediate raw material.
In such a process called high pressure acid leach (HPAL), nickel can be almost completely leached even from low-grade ore in which valuable metals intended for recovery are contained by not more than 1% to 2% by weight (hereinafter indicated by “%” with regard to a grade). Further, the HPAL process has a feature of concentrating the valuable metals up to the same grade as a conventional raw material by producing an intermediate raw material from a leachate, and refining the nickel in a process similar to a conventional process.
Further, the HPAL process may be applied to various types of ores such as nickel sulfide ore, copper sulfide ore, and copper oxide ore, in addition to the nickel oxide ore.
Further, a main component of the leach residue obtained by the HPAL process is iron oxide having the form of hematite. This is secondarily obtained because each of oxide ore and sulfide ore of nickel or copper used as a raw material contains iron of an amount far more than a content of nickel or copper.
These leach residues are created at a high temperature, and thus have the form of oxide that is chemically or environmentally stable. However, the leach residues have no special utility value, and have been scrapped to a residue disposal yard. For this reason, it has been a grave challenge how to secure the disposal yards for an enormous amount of leach residues generated along with the smelting.
Furthermore, the leach residue of the HPAL process cannot be directly used for the aforementioned iron-making raw material. The reason is that the leach residue of the HPAL process contains gangue and impurities, particularly sulfur, in addition to the iron oxide and requires exhaust gas treatment, and thus is not suitable for the raw material used in the conventional iron-making process in common.
Particularly, a grade of sulfur in iron oxide usable for the iron-making raw material differs depending on facility capacity and an amount of production of individual ironworks, and generally needs to be suppressed to less than 1%.
The sulfur is hardly contained in the original nickel oxide ore. Nevertheless, the sulfur contained in the leach residue by about 1 to 3% results from calcium sulfate (plaster) generated by reaction of sulfuric acid and limestone or slaked lime added as the neutralizer in order to neutralize free sulfuric acid remaining at the leach slurry.
Therefore, it is considered that what creates a soluble salt may be used as the added neutralizer, not the slaked lime or what forms insoluble sediment, such as the slaked lime, after the neutralization.
For example, the neutralizer suitable for such application includes sodium hydroxide, potassium hydroxide, magnesium hydroxide, and magnesium oxide.
However, these neutralizers are expensive, and have a limited amount of production. Thus, when a large quantity of neutralizer is required as in the HPAL process, it is industrially difficult to cover the whole quantity.
For this reason, there has been no choice but to use a calcium-based neutralizer in whole or in part which forms the insoluble sediment after the neutralization as described above, and thereby mixing of the sulfur has been inevitable. As such, it has been impossible to process the leach residue created in the HPAL process into the hematite and to use it as the iron-making raw material.
On the other hand, a method of separating sulfur in jarosite using a pressure device such as an autoclave is also known.
For example, JP H03-176081 A discloses a method that includes stirring a jarosite-containing residual and a zinc sulfide inclusion in an autoclave at least under oxygen partial pressure of 1000 kPa at a temperature of 130 to 170° C. along with a free sulfuric acid of 40 to 100 g/l, substantially dissolving iron and zinc fractions of a concentrate containing the residual and zinc sulfide, introducing the solution into a leach circulation passage for zinc electrolysis to settle iron in the form of hematite, and separating sulfur from the above solid, and supplying the residual for separate application.
However, this method requires an expensive device such as an autoclave, increases a facility cost, and further has a problem even in the aspect of productivity.
The invention is intended to provide a production method for refining hematite, which has such a low sulfur component as to be used as an iron-making raw material, from a leach residue containing iron oxide produced by a high pressure acid leach (HPAL) process.